This invention relates to normally tacky and pressure-sensitive adhesive (PSA) compositions and to PSA tapes made therewith. The invention is particularly concerned with PSA products capable of adhering firmly to plasticized vinyl substrates, both initially and after contact over an extended period of time.
Polyvinyl chloride (or, as it is more commonly referred to, "vinyl") is one of the most versatile and widely used synthetic polymers available today. In its unmodified form, it is rigid and capable of being fabricated into containers, structural components, tubing, etc. When compounded with a plasticizer, it can be calendered into flexible sheet material that can be used as upholstery fabric, floor covering, PSA tape backing, rainwear, etc., or extruded as electrical insulation, auto body side moulding, etc. Plasticized vinyls typically contain 15-50% by weight of either a mononeric or a polymeric plasticizer; because the monomeric plasticizers are less expensive, they are more commonly employed. Unfortunately, however, the monomeric plasticizers are of relatively low molecular weight, tending not only to migrate to the surface of the plasticized vinyls but also to volatilize therefrom, gradually embrittling the originally flexible product.
The tendency of plasticizer to migrate into materials with which it comes in contact is also well known; see, e.g., U.S. Pat. No. 4,284,681. Thus, when a PSA is applied to a plasticized vinyl substrate and allowed to remain in contact with it for an extended period of time, plasticizer from the vinyl tends to migrate into the PSA, softening it and causing a decrease in adhesion; see, e.g., U.K. Pat. No. 1,051,125.
Attempts have been made to solve the plasticizer migration problem by interposing an impenetrable barrier between the vinyl and the PSA; see, e.g., U.S. Pat. Nos. 4,045,600 and 4,605,592. Others have developed PSAs that are said to be less susceptible to weakening by plasticizer; see, e.g., Graziano et al., Development of Acrylic Latex Pressure Sensitive Adhesive for Plasticized PVC films, Journal of Plastic Film & Sheeting, Vol. 2, April 1986, pp. 95-110. A commercially available tape that is widely used for adhering to plasticized vinyl utilizes a PSA that is a terpolymer of 2-ethyl hexyl acrylate, vinyl acetate, and ethyl acrylate.
Another attempt to prevent migration of plasticizer from a plasticized vinyl substrate into a confronting PSA has been to incorporate plasticizer into the PSA, thereby minimizing the plasticizer gradient between the contacting layers; see, e.g., U.S. Pat. No. 4,272,573, U.K. Pat. App. No. 2,162,191-A, and European Pat. App. No. 150,978.
Despite the limited success achieved by products of the type discussed in preceding paragraphs, there has remained a strong commercial desire for a PSA that was more effective, particularly one that not only initially adhered strongly to plasticized vinyl but which also retained a high degree of adhesion after remaining in contact with it for extended periods of time. In particular, none of the prior art products have been able to provide excellent resistance to shear after prolonged contact with plasticized vinyl.